Optimisation of the ozone pre-treatment of agricultural residues and conversion to platform chemicals

Abstract

Biomass in the form of agricultural residues offers clear potential for conversion into energy, especially the use of processing residues such as corn cob and rice husk etc, due to their abundance/availability and high carbohydrate content. Depending on the conversion process, pre-treatment allows easier access to the carbohydrate components (cellulose and hemicellulose) for conversion to advanced generation biofuels and platform chemicals such as 5-hydroxymethyl furfural, furfural etc. leaving lignin as a low-value residue which is often burnt to provide parasitic energy for the conversion process. The conversion of lignin into high value platform chemicals will increase the profitability and sustainability of the biorefinery process. However, to achieve complete utilisation of the lignin, it must be of high purity and lack extensive modifications. Ozone pre-treatment is recently gaining attention as a promising green alternative for cellulose isolation. However, the drawback of using ozone for large-scale industrial applications is the high costs for generating ozone in-situ as it cannot be stored due to its short lifetime and high reactivity. This study aims to pre-treat and fractionate the agricultural processing residues (corn cob and spelt husk) and convert the carbohydrate components to chemical platforms (5-hydroxymethyl furfural and furfural) that can be used in several applications i.e medicines, diesel, fuel additives and plastics. The first stage of this study developed an energy-efficient surface two-zone plasma ozone generator consisting of two stainless-steel mesh electrodes and a dielectric of quartz glass. The design offered good temperature control, which produced 2.5 times higher concentration and quantity of ozone at the same power input than a conventional single-zone plasma reactor. A maximum ozone concentration of 140g m-3 and 90g (kWh) -1 productivity was obtained from the two-zone system, comparable to commercial ozone generators but with 30 – 40% lower power consumption (11kWh kg-1 O3). Hence mitigating the drawbacks with the use of ozone in industrial applications caused by large energy demand. Optimisation of the ozone pre-treatment process was achieved by incorporating ultrasound which enhanced lignin separation by 38.5%. Following organosolv fractionation at low temperature (80⁰C), about 90% and 94% of lignin with high purity (95%) were recovered for corn cob and spelt husk respectively with guaiacyl-syringyl lignin the major fraction from corn cob and guaiacyl lignin from spelt husk. In addition, 84 - 85% cellulose was recovered with III 78% purity. The recovered cellulose had its crystallinity decreased by 19% and its degree of polymerisation (DP) decreased by 17%. In a microwave reactor, corn cob and spelt husk (untreated and pre-treated) were reacted in a DMSO-H2O media to produce HMF and furfural. HMF and furfural yields of untreated corn cob were higher than those from spelt husk due to a difference in their morphology with increased porosity of corn cob allowing easy access to cellulose. Pre-treatment led to a 58% and 74% increase in HMF yield for corn cob and spelt husk respectively, while a 10% and 66.7% increase in furfural. Reacting fractionated cellulose from corn cob and spelt husk yielded a similar HMF yield of 40mg g -1 irrespective of the feedstock used. Overall, HMF and furfural yields were influenced by changes in cellulose properties following pre-treatment such as fibre size, increased surface area, decrease in the degree of polymerisation and decrease in lignin content following lignocellulose pre-treatment. The economic efficiency and competitiveness of the conversion process for large scale co production of HMF, furfural and lignin from spelt husk was determined. The proposed approach was compared with process where HMF and furfural were co-produced without lignin. Co-production with lignin yielded a profit of $213,657 higher than without lignin for a plant size of 100 tonnes per day of spelt husk due to extra revenue obtained from lignin sales, meaning fractionation of lignin had a positive effect on the process economics than its utilisation for heat or boiler fuel. Thus, the improved pre-treatment and quality separation of biomass components enhanced downstream conversion to value-added products, thereby improving the sustainability and cost-effectiveness of the ozone pre-treatment process and conversion to HMF and furfural. In addition, the co-production of lignin will offset the cost of production of platform chemicals, thereby increasing the economics of a biorefinery.